Plasma treatments are often used in the manufacture of such electronic devices. The plasma treatment is carried out on a device substrate which is mounted on a supporting electrode facing a perforated gas-feeding electrode in the apparatus. A reactive plasma is generated in the space between the electrodes from a mixture of reaction gases which is fed into the space through at least the perforated electrode. From the English language abstract of published Japanese Patent Application Kokai JP-A-56-87328 in Patent Abstracts of Japan and the drawings of JP-A-56-87328 (hereby incorporated herein as reference material), such a method and apparatus are know in which the mixture of gases flows in a direction across the substrate from a first area of the space to which it is supplied by a first supply line, and a second supply line feeds a second area of the space through the perforated electrode; the second area is along the flow direction from the first area.
In the arrangement of JP-A-56-87328, the gas mixture is supplied to the perforated electrode by the first supply line feeding a central area of the perforated electrode and by the second supply line feeding a peripheral area of the perforated electrode. The gas flow over the substrate is radially outwards from the central area. In this known apparatus and method the same gas mixture is fed to both the peripheral and central areas, but at different rates which are controlled by a first mass flow meter on the first supply line and a second mass flow meter on the second supply line. This permits a reduction of non-uniformities in the plasma between the central area and the peripheral area. Such a plasma treatment may be used for the deposition of a silicon nitride film, for example from a mixture of silane (SiH.sub.4) and ammonia (NH.sub.3).
Plasmas used in processing treatments are extremely complex, and many of the details of physical and chemical interactions both within the plasma and with surfaces exposed to the plasma are not yet understood. This situation pertains in even the well known silicon nitride deposition process from SiH.sub.4 and NH.sub.3. The present inventors find that, in spite of adjusting different flow rates in the peripheral and central areas, significant non-uniformities can still occur in the thickness, composition and quality of the deposited film. The non-uniformities are especially noticeable when the film is deposited over large areas, for example with electrode areas of about 0.5 m.sup.2 (meter.sup.2) and more, e.g of the order of 1 m.sup.2 (meter.sup.2). Similar non-uniformities can occur in other large-area plasma treatments, for example in plasma etching treatments using mixtures of reaction gases for etching over large areas.